1. Field of the Invention
This invention relates to dry stretched polyolefin (PP, PE, and PP/PE/PP) microporous membranes as battery separators, specifically to the continuous method of making the membrane separators.
2. Discussion of Prior Art
In batteries, the anode and cathode are separated from one another by a separator. Today, “lithium batteries” are very popular because they are able to generate high-energy outputs. The lithium battery market can be divided into two groups, the “primary” lithium battery and the “secondary” lithium battery. The primary lithium battery is a disposable battery, while the secondary lithium battery is a rechargeable battery.
The rechargeable lithium battery technology was first commercialized by Sony in 1992. Since then, this new type of high-energy rechargeable batteries has been widely used in consumer markets, such as computer, camcorder and cellular phone. More applications are being developed. One of them is the Electric Vehicle (EV) and the Hybrid Electric Vehicle (HEV) applications.
However, the rechargeable lithium batteries for EV and HEV applications are significantly larger than the consumer rechargeable lithium batteries. The separator used in the batteries needs much more square footage than that in the consumer batteries. In the EV and HEV batteries, the unit cost of the separator needs to be much lower to make the EV and HEV batteries commercially possible. In addition, the cost of the separator in non-rechargeable lithium battery remains relatively high in comparison to that of the separators in other types of batteries.
In summary, there is a strong need for low-cost separators for lithium batteries, especially for rechargeable lithium battery.
The separators for lithium batteries can be made from polyolefin basically with two types of processes in the prior art: dry-stretch process and solvent-stretch process. The solvent-stretch process (such as U.S. Pat. Nos. 4,539,256; 4,726,989) usually costs more than the dry-stretch process, and it creates environmental issues. It is not our interest. Our focus is on the clean dry-stretch process for the micro-porous membranes as separators for the lithium batteries.
Polyolefin, as used herein, refers to a class or group name for thermoplastic polymers derived from simple olefins. Exemplary polyolefins include polyethylene and polypropylene. Polyethylene refers to, for example, polymers and copolymer substantially consisting of ethylene monomers. Polypropylene refers to, for example, polymers and copolymers substantially consisting of propylene monomers.
Developed in the prior art are single-ply and multiply of dry-stretch microporous membrane from polyolefin resins, including polypropylene (PP) and polyethylene (PE), with at least three separate steps: (a) film extrusion (blown film or slit film), (b) annealing, and (c) stretching, as described in U.S. Pat. Nos. 3,426,754; 3,558,764; 3,679,538; 3,801,404; 3,801,692; 3,843,761; 4,138,459; 4,994,335 and 5,173,235.
The term, multiply, herein is defined as more than one ply of film or membrane stacked together and then rolled up into a big roll. The adhesion between plies is minimum, and the multiply films or membranes can be easily deplied into multiple single-ply film or membrane. So, multiply can be two-ply, four-ply, or eight-ply. In contrast, the term, multi-layer, herein is defined as more than one layer of film adhered together with a reasonably good adhesion. The multi-layer film (or membrane) can be handled as a single-ply film (or membrane). So, multi-layer can be bilayer, trilayer, penta-layer. For example, PP/PE/PP trilayer membrane means that the two layers of PP membrane sandwich one layer of PE membrane with a reasonably good adhesion, and that the trilayer can be handled as a single-ply membrane without losing its integrity.
The terms, microporous membrane and membrane, herein imply the open-cell microporous membrane.
In the prior art, U.S. Pat. No. 3,801,692, a cold stretching was applied right before hot stretching, and the stretched membrane gave a higher porosity.
In the prior art, U.S. Pat. No. 3,843,761, right after cold stretching, the precursor film was hot stretched in a plurality of discrete hot stretching increments. The obtained stretched microporous membrane had greater gas flux.
In the prior art, U.S. Pat. No. 4,058,582, more than two plies of precursor film were simultaneously stretched for the first time. The surface properties of microporous membranes were significantly improved.
In the prior art, U.S. Pat. No. 4,138,459, an additional heat relaxing step was added to the end of the hot stretching process, and the dimensional stability of the final membrane was improved with a significantly lower shrinkage.
In the recent multi-layer products (PP/PE/PP trilayer and PP/PE bilayer separators) as described in U.S. Pat. Nos. 5,565,281; 5,691,077; 5,952,120, the process also includes three separate steps: film extrusion, annealing and stretching. The extruded films are rolled up first into big film rolls, and the film rolls are then unrolled and fed to an oven for annealing. The annealed films are rolled up into multiply big rolls, and the annealed rolls are then unrolled and fed to another oven for stretching into membranes. The stretched membranes from the oven are rolled up into rolls with desired length and filter sent for deplying and slitting separately.
The disadvantages of the conventional process with separate steps are the following:                1. Yield loss due to many more start-ups and endings of the film rolls.        2. Potentially less stable product quality due to the start-up and ending of each step        3. More oven idle time during the change of film rolls        4. More space and film-roll/inventory management is needed for the intermediate products. It adds extra operation cost.        5. Need more manpower for more separate steps of operation. It adds extra operation cost.        6. Potentially more maintenance needed due to much more frequently machine on and off, especially with the ovens for annealing and stretching.        
As described in U.S. Pat. Nos. 3,426,754; 3,558,764; 3,679,538; 3,801,404; 3,801,692; 3,843,761; 4,138,459; 4,994,335 and 5,173,235, the films (either blown film extrusion or slit film extrusion) was extruded with the state-of-the-art film extrusion machine technology for packaging films, which require as fast as possible. However, in the process for the dry stretch membranes, the fast film extrusion speed is not compatible with rest of the processes in ovens. It made the overall process break into three separate steps, which are undesirable. Even though the state-of-the-art film extrusion line can be run faster for multi-rolls for the followed separate processes in the oven, the obtained precursor film and membranes lost some products during start-up and ending in the separate oven processes. It lost the production efficiency.
Accordingly, there is a strong need to reinvent a process for low-cost membrane separators.